The present invention relates to a liquidxe2x80x94liquid extraction device for extraction of at least one analyte from a sample and on-line transfer of said analyte(s) to a gas chromatography apparatus, and to a method in which the extraction device is used.
In the preparation of samples before gas chromatography analysis, several techniques are known. One of the most suitable techniques used at present is an LLE (liquidxe2x80x94liquid extraction) operation before the gas chromatography analysis. However, this technique is associated with several disadvantages. First, large volumes of the solvent for extraction are required, which is hazardous for the operator and the environment. Further, this technique is expensive, the operations are laborious and automatisation is difficult. Moreover, this technique rarely allows for anything but off-line operations with a concomitant risk of sample contamination. Emulsion formation and waste problems are also associated with this technique, as well as difficulties in handling small sample volumes.
Another useful technique is SPE (Solid Phase Extraction). One of the major problems of this technique is that it includes a complicated phase exchange before the gas chromatography (GC) analysis. Another problem is that it suffers from break-through and this effect is more or less pronounced depending on the sample matrix. Moreover, the SPE technique is associated with the same problems as the LLE technique, even though the emulsion formation problems and the automatisation difficulties are not as pronounced for the SPE technique.
A fairly new technique is SPME (Solid Phase Micro Extraction) coupled to gas chromatography. Although the SPME technique is considered to be increasingly utilized in the future, it suffers from several shortcomings. In this technique, a rigid glass fibre, either bare or covered with a suitable stationary phase, is contacted with the sample in the extraction step. This technique involves several phase conversions, i.e. from gas to liquid and then to gas again. Thus, the analytes are distributed between the sample liquid and the air gap above the sample where the sampling takes place (head-space analysis). With a view to increasing the amount of analytes in the gas phase, the sample is often heated. Subsequently, the analytes are distributed between the gas phase and the liquid on the glass fibre. Further, the glass fibre has to be transferred to the GC injector after the extraction step. When the glass fibre is inserted in the injector, the analytes are to be desorbed from the fibre and transferred to the gas phase without the evaporation of the extracting medium (the liquid). This is often done manually, but can be automatised. The desorption operation must preferably take place very quickly. In cases where the SPME glass fibre is used for extraction into an aqueous medium, the fibre is dipped in the sample solution which is shaken, or the fibre is rotated.
A further disadvantage of the SPME technique is that the extraction unit, i.e. the fibre covered with liquid and enclosed in a syringe, is not stationarily enclosed in the syringe and has to be handled outside the analysis system during the analysis.
The SPME technique is also time-consuming and the time period until equilibrium can amount to several hours. This technique is also highly sample-matrix dependent, which can reduce the analyte capacity and causes a risk of fouling of the stationary phase. Also, the equilibrium constants may vary between samples. Problems have also been encountered with the reproducibility, the repeatability (RSD 1-92%) and the linearity (fouling and memory effects). Further, the fibres are mechanically and thermally sensitive, and air bubbles-may occur at the surface when the liquids are analysed. The SPME technique is also relatively expensive.
Still another extraction technique is MMLLE (Microporous Membrane Liquidxe2x80x94Liquid Extraction). This technique resembles the technique according to the present invention, but differs in several important aspects. The organic liquid in the MMLLE technique is continuously moving during the extraction step. Further, the sample injection volume is very large. MMLLE C necessitates additional extraction hardware as it involves two liquid chromatography injectors and an intermediate transfer of the extract (organic liquid containing analytes) to a loop, thereby causing a more dispersed (diluted) sample, and potential analyte losses, and it makes handling of small sample volumes impossible. Moreover, a so-called xe2x80x9cretention gapxe2x80x9d complicates the GC analysis, i.e. the gas chromatograph has to be modified in some respects when applied to the MMLLE technique. Finally, the transfer of the extract in this technique is performed by a gas pressure being exerted by the support gas flow in the gas chromatograph which is another GC modification.
The object of the present invention is to solve the above-mentioned problems described in connection with the background art.
This object is achieved by a liquidxe2x80x94liquid extraction device and a method for extraction of the type described by way of introduction and which are further defined in the characterising part of claim 1 and the independent method claim. Other embodiments are defined in the subclaims. Other objects and advantages will be apparent from the detailed description of the invention and the appended claims.